Self-tapping fasteners that form threads by deforming a thread pattern within a pilot hole have become increasingly popular. There are many advantages to using a roll-forming thread, rather than a more-traditional thread-cutting design. A very successful style of roll-forming thread is the multiple-lobed fastener shown, for example, in Phipard, U.S. Pat. Nos. 3,195,156 and 3,918,345, the contents of which are incorporated herein by reference. This fastener is available from a variety of sources carrying the trademark TRILOBULAR, in connection with a three-lobe thread-forming blank design.
FIGS. 1-5 illustrate a conventional three-lobed thread-forming blank 10 and resulting fastener (21) according to the prior art. In manufacturing a threaded fastener, a blank 10 is first provided with a head 12 as detailed. The head, 12 in this example is a standard hex washer head design. However any head formation enabling the driving of the fastener in a rotary fashion about a central axis 14 can be formed. The head is formed by striking the end of a straight blank "wire" segment using header punches that forcibly deform one end of a metallic wire made of steel or another hard metal. As will be described further a header die deforms a specialized tapered thread-forming and stabilizing section on the opposite end (the "tip") of the shaft. The wire, at the time of head and tip formation, typically already has a lobular cross-section 16 as revealed in FIG. 3. Each lobe 18 is, in essence, an "out-of-round" formation. In other words, the lobes present an eccentric cross section with respect to an otherwise round shape. In fact, formation of the blank, prior to heading involves the drawing of a round-cross-section wire through a lobed female die to create the illustrated out-of-round on the main body of the blank (10). The headed blank 10 is passed between a pair of forming dies to form thereon a set of external threads 22 detailed generally in FIG. 4. These threads exhibit the out-of-round characteristics of the blank. In general, the external apices 24 of each thread are located at an outer diameter greater than that of the original blank, since material has been displaced from the thread troughs 26 outwardly at predetermined locations to form each apex.
All threads have a characteristic pitch and diameter. Because of the lobulation of the threads, the radial offset from the axis 14 will vary about the circumference. In general, standard thread diameters and pitches are provided to lobular fasteners, but the lobes tend to have a slightly larger diameter than a standard thread diameter. This enables the lobes to positively form corresponding internal threads as the fastener is driven into an appropriately-sized pilot hole into the shape of conforming internal threads.
In other words, as the fastener 21 (FIGS. 4 and 5) is rotated clockwise, the lobes engage the inner wall of the pilot hole (not shown) and begin to displace material within the pilot hole. The threaded fastener 21 is provided with a discrete stabilizing zone 30 having stabilizing threads 32, and a thread-forming zone 33 with corresponding thread-forming threads 34. The stabilizing zone 30, as detailed in FIGS. 1 and 2, has a reduced diameter, enabling it to fit within an untapped pilot hole in a relatively perpendicular fashion. The thread-forming zone 33 has a sloped/tapered shape with diameter that increases linearly between the stabilizing zone and the full-diameter main body 20. Note that the thread stabilizing zone 30 has a higher out-of-round than the main body 20. This is clear from the end-on view of FIG. 2. The thread forming zone 33 also has a higher out-of-round than the full-diameter main body 20. In one example, the out-of-round of the thread forming zone gradually tapers back from the highest out-of-round adjacent to the stabilizing zone toward the lower out-of-round that defines the full-diameter main body 20. In another, usually preferred example, the thread forming zone can define an approximately constant-profile high out-of-round along its entire axial length, that transitions stepwise at the main body into the characteristic lower out-of-round. In connection with either example, note the difference between the high out-of-round at the stabilizing section end (FIG. 2) and at the main body cross section (FIG. 3).
As a fastener is driven into an untapped pilot hole, the thread-forming threads 34 encounter the sidewalls of the hole initially. These threads exhibit an increasing outer diameter and higher out-of-round. As such, the lobes are able to gradually apply increasing thread-forming pressure to the pilot hole until each formed internal thread is contacted by the first full-diameter thread 40. This first full-diameter thread 40 has the out-of-round profile of the rest of the main body. It provides the final formation of each thread in the pilot hole to the desired shape.
In general, to form a thread properly requires several swaging "blows" from the underlying lobes of the fastener. This process is, in essence, similar to the formation of a shape in a ductile metal by a blacksmith's hammer. A sufficient number of forming threads is necessary to complete the process. To make the process go more quickly, a high out-of-round, which concentrates the force of the blows is desirable. Use of a high out-of-round within the main, full-diameter threads substantially reduces the amount of torque that must be applied to form threads. However, this lower torque comes at a price, since it results in less diametrical material remaining in contact with the internal thread once it is formed. Hence, it is more likely that failure will occur in such a fastener system such failure, in general, results from axial pull-out or, when thicker nut members are used, fracture. Also, since area varies by the square of the radius, the use of a higher out-of-round cross section results in a significantly reduced cross sectional area, which lowers the screw's failure limit. Hence, self-tapping screws typically use an out-of-round dimension that is a compromise between the optimum value for thread forming efficiency and the optimum for resistance to failure.
Previous attempts to balance a desire for higher out-of-round, versus failure resistance is described in U.S. Pat. No. 4,040,328 to Muenchinger. This patent provides a tapered point zone on the fastener with stabilizing threads that have a higher out-of-round than the fastener body. However, the out-of-round of the stabilizing threads is, in fact, significantly greater than that of the full-forming thread. This is a less efficient structure, since the stabilizing and thread-forming threads are tasked with virtually all the forming work, while the first full-diameter thread has the lower out-of-round cross section, which is inefficient for thread forming purposes.
It is therefore an object of this invention to provide a fastener blank, and under-lying threaded fastener formed therefrom, that exhibits reasonable low thread-forming torque, while maintaining good resistance to failure, once it is in engagement within a nut-member or other thread-formed hole.